Various techniques are known in the art for determining the location of a signal source by receiving and processing the signal emitted from the source by a plurality of signal receivers.
For example, one such technique, known as Frequency Difference of Arrival (FDOA), also known as Differential Doppler (DD), provides for estimating the location of a radio signal emitter based on observations/receipt of the signal from other points/receivers. In the FDOA technique the receivers must be in relative motion with respect to the signal emitter. For example, in geolocation applications, the relative motion is sometimes achieved by using airborne receivers on an aircraft. This relative motion between the receivers results in different Doppler shifts in the signals that are received by the receivers. The location of the signal source is then estimated based on the locations and velocities of the receivers and the Doppler shifts in the signals received by the receivers.
A known disadvantage of the FDOA technique is that large amounts of data must be moved between receivers to a central location (processing center) to carry out the cross-correlation that is necessary to estimate the Doppler shift. The accuracy of the location estimate is related to the signal-to-noise ratio at each receiver point, and the geometry and vector velocities of the receivers points. Also, it is difficult to measure frequency of pulse-type signals to the level of accuracy needed to carry out the FDOA technique, because the frequency resolution is equal to 1/T, where T is the pulse duration.
Time Difference of Arrival (TDOA, also known as multilateration) is another known technique for determining the location of a signal source by a plurality of signal receivers. TDOA takes advantage of the fact that a transmitted signal will arrive at different times to receivers at different locations. According to this technique, a number of spatially separated receivers capture the emitted signal, and the time differences of the arrival (TDOAs) of the signal to the receivers are determined. The emitter's location is calculated by using the TDOAs and the location of the receivers.
When using the TDOA technique, the receivers and the emitter may be stationary, however the signal emitted from the emitter should generally be modulated (e.g., pulsed) to thereby enable identifying and measuring the time of arrival (TOA) of the modulation pattern to the different receivers, and determining the DTOAs of the modulation pattern in between different receivers. To this end, the TDOA technique requires that the two or more geographically separated receivers will be time synchronized with each other, in order to allow precise measurement of the TDOAs providing for determining the location of the emitter.